1. Field of the Invention
The present invention relates to solid state image sensors, and more particularly to solid state image sensors capable of preventing degradation in image quality at dark areas relative to light areas upon photographing an object including the dark and light areas.
2. Description of the Related Art
Generally, solid state image sensors include a plurality of photoelectric conversion elements arranged in a matrix manner. Each photoelectric conversion element corresponds to each pixel and generates a signal charge proportional to the quantity of incident light. A pixel signal generated by each photoelectric conversion element is extracted via a metal oxide semiconductor (MOS) transistor, so as to pick up an image of an object.
Such solid state image sensors are classified into two types, one type being a charge coupled device (CCD) type and the other type being a MOS type. The MOS type solid state image sensor uses a photodiode as each source region thereof.
The CCD type solid state image sensor has a photosensor disposed at the lower surface of its electrode. On the other hand, the MOS type solid state image sensor has a photosensor disposed in flush with a switch and a signal output line. Such photosensors can be manufactured by using a MOS process technique for achieving a high integration.
The present invention is directed to a solid state image sensor of the NOS type constituted by a combination of a photodiode and a switching MOS transistor.
Referring to FIGS. 1 to 5, there is illustrated a MOS type color image sensor as an example of a conventional solid state image sensor. FIG. 1 is a circuit diagram of the conventional solid state image sensor. As shown in FIG. 1, the solid state image sensor which is denoted by the reference numeral 10 mainly includes a vertical charge transfer unit 11 for generating vertical scanning signals VS1 to VS4 in response to input signals V1 and V2, a horizontal charge transfer unit 12 for generating horizontal scanning signals HS1 to HS4 in response to input signals H1 and H2, and a photoelectric conversion unit 13 including a plurality of photoelectric conversion elements 100 arranged in a matrix manner, each photoelectric conversion element having a photodiode PD and MOS transistors M1 and M2 controllably switched by the vertical and horizontal scanning signals rS1 to rS4 and HS1 to HS4.
Each photoelectric conversion element 100 of the photoelectric conversion unit 13 corresponds to each pixel and serves as a pixel cell for generating a charge proportional to the quantity of light incident on each corresponding pixel, so as to pick up an image of an object. The MOS transistor M1 of each pixel cell has a gate to which vertical scanning signals VS1 to VS4 are applied. On the other hand, the MOS transistor M2 has a gate to which horizontal scanning signals HS1 to HS4 are applied.
In FIG. 1, the photoelectric conversion unit 13 illustrated as having pixel cells arranged in 4 columns and 4 rows. As shown in FIG. 1, horizontal scanning signals HS1 to HS4 outputted from the horizontal charge transfer unit 12 are applied to gate terminals of MOS transistors M2 of photoelectric conversion elements 100 arranged transversely in each row, respectively. On the other hand, vertical scanning signals VS1 to rS4 outputted from the vertical charge transfer unit 11 are applied to gate terminals of MOS transistors M1 of photoelectric conversion elements 100 arranged longitudinally in each column, respectively. The photoelectric conversion unit 13 also includes switching MOS transistors 14 for selecting a vertical position of each corresponding pixel. The switching MOS transistors 14 are coupled in common to an output terminal OUT of the photoelectric conversion unit 13
FIG. 2 is a sectional view of each photoelectric conversion element 100 corresponding to one pixel in the photoelectric conversion unit 13 shown in FIG. 1. FIG. 3 is a circuit diagram of an equivalent circuit of the photoelectric conversion element shown in FIG. 2.
In the photoelectric conversion element 100 corresponding to one pixel, the photodiode PD grounded at its anode is coupled to the source of the MOS transistor M1, as shown in FIG. 3. To the gate G1 of the MOS transistor M1 is applied a vertical scanning signal, for example, VS1, from the vertical charge transfer unit 11. The MOS transistor M1 is also coupled at its drain to the source of the MOS transistor M2. The MOS transistor M2 receives at its gate G2 a horizontal scanning signal, for example, HS1, from the horizontal charge transfer unit 12 and is coupled at its drain to the output terminal OUT of the photoelectric conversion unit 13.
Now, the structure of the photoelectric conversion element 100 having the above-mentioned equivalent circuit will be described in conjunction with FIG. 2.
The photoelectric conversion element 100 includes a N type substrate 21 and a P type well 22 formed on the N type substrate 21 and serving as a drain of the MOS transistor M1 and a source of the MOS transistor M2. In the P type well 22, a first impurity region 23 of the N type serving as a cathode of the photodiode PD and a source of the MOS transistor MI, a second impurity region 24 of the N type serving as a drain of the MOS transistor M1 and a source of the MOS transistor M2, and a third impurity region 25 of the N type serving as a drain of the MOS transistor M2. The gate G1 of MOS transistor M1 is formed between the first impurity region 23 and the second impurity region 24 above the N-type substrate 21 and insulated by an oxide film 26 formed over the P type well 22. In a similar manner, the gate G2 of MOS transistor M2 is formed between the second impurity region 24 and the third impurity region 25 above the N-type substrate 21 and insulated by the oxide film 26. Over the oxide film 26 is formed an insulating phosphorous silicate glass (PSG) film 27 which is, in turn, covered with a color filter layer 28 surrounded by a passive film 29.
A procedure that the solid state image sensor with the above-mentioned structure scan an object and generate video signals will now be described.
First, as light enters each photodiode PD, a charge proportional to the quantity of the incident light is generated and then accumulated in the photodiode PD. This phenomenon occurs simultaneously at all photodiodes PD of photoelectric conversion elements. Totally, the quantities of signal charges accumulated in photodiodes PD are proportional to the luminosities of incident images through a lens, respectively.
After completing the charge accumulation in photodiodes PD, the charges accumulated in the photodiodes PD are read out through the switching MOS transistors M1 and M2 coupled to specific ones of a plurality of pixels according to control signals H1, H2, Hin, V1, V2 and Vin, so that a video signal of one field is obtained.
Referring to FIG. 4, there is illustrated an example of a camera system utilizing such a solid state image sensor. As shown in FIG. 4, the camera system includes a lens 41, an iris 42, a motor 43 adapted to open and close the iris 42, an iris control unit 44 adapted to detect the average illuminance of the picked-up image and drive the motor 43 so that the iris 42 is opened or closed properly to make the average illuminance correspond to a predetermined value, and an automatic gain control (AGC) unit 45 adapted to operate when a video signal generated after the control operation of the iris control unit 44 is at a level not higher than a predetermined illuminance level, so as to provide a proper gain.
Generally, the illuminance in the natural world in which most objects for video cameras are presented ranges from 10.sup.-4 1x (Lux) near darkness to 10.sup.5 1x of direct rays of sun light. On the other hand, the photodiodes have a dynamic range of a 10.sup.1 grade due to limitations such as characteristics of materials used and the size of a pixel. Because such image sensors have difficulties in directly picking up an image of an object of the natural world, the existing camera systems utilize control devices such as the iris control unit 44 or the AGC unit 45, so as to cope with the broad range of the illuminance in the natural world.
However, the basic assumption in such systems is that objects in the natural world are presented within a specific range of the illuminance. In other words, images in places such as a desert and a beach are presented within a high illuminance range above a predetermined value. On the other hand, the images in places such as rooms are presented within a lower illuminance range. As shown in FIG. 5, objects at a beach can be controlled to cope with the dynamic range of an image sensor, by reducing totally the illuminance of the images being picked up by the image sensor. In the existing camera systems, such a control function is achieved only by the iris 42. In a dark room, a totally dark image is obtained even when the iris is fully opened. In this case, accordingly, the AGC unit 45 operates to increase the gain for a video signal so that the video signal has a proper illuminance. The above-mentioned camera systems can exhibit a superior performance for objects whose images are presented within a specific illuminance range.
However, the illuminance condition of objects presented in the natural world is considerably various, beyond the cases mentioned above. For example, there is a back light condition. The back light condition is the case that a part of an image is very light.
In such a back light condition, an image includes both a low illuminance condition, for example, in a room and a high illuminance condition, for example, of sun light or other light sources. In this case, it can be found that the illuminance range of an object exceeds considerably the dynamic range of the image sensor. Under this condition, a severe degradation in image quality occurs at the low illuminance part of the image due to limitations in the photoelectric conversion elements, although the iris 42 and the AGC unit 45 control properly the quantity of light incident on the image sensor. Such a problem cannot be solved by the existing systems wherein a control value for the quantity of light is calculated based on the average illuminance of an object, although the existing image sensors have an illuminance condition capable of obtaining a sufficiently good image by virtue of efforts to develop image sensors of a high sensitivity.